1. Field of Invention
The present invention relates generally to a hybrid electric vehicle (HEV), and specifically to a method and system to control an HEV engine shutdown.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Unfortunately, electric vehicles have limited range and limited power capabilities. Further, electric vehicles need substantial time to recharge their batteries. An alternative solution is to combine both an ICE and electric traction motor into one vehicle. Such vehicles are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that together provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is typically known as a “powersplit” configuration. In the PSHEV, the ICE is mechanically coupled to two electric motors in a planetary gearset transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque powers the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery if a regenerative braking system is used.
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing the fuel consumption and emissions of the ICE with no appreciable loss of vehicle performance or driveability. Nevertheless, new ways must be developed to optimize these HEV potential benefits.
One such area of HEV development is implementing a controlled engine shutdown in an HEV. If the engine shuts down in an uncontrolled manner, its starts and stops throughout a given HEV drive cycle can increase tailpipe emissions from inconsistent amounts of residual fuel (vapor and puddles) in the intake manifold from one shutdown to the next. The amount of residual fuel depends on the amount of liquid fuel flow from the injectors, as well as the amount of fuel vapor introduced by the vapor management valve (VMV) and exhaust gas recirculation valve (EGR) prior to the shutdown.
Vapor management valves (VMV) are widely used in evaporative emission control systems to reduce the fuel vapor build up in the fuel system. Fuel vapor in the fuel tank and lines is captured in a vapor storage canister (typically a charcoal material), and then drawn out into the engine's intake manifold via the VMV. The amount of fuel vapor introduced into the intake manifold, and thus into the engine cylinders to be combusted, is proportional to how much the VMV is opened and how much intake manifold vacuum is available.
Exhaust gas recirculation valves (EGR) are widely used in tailpipe emission control systems to re-circulate a portion of the hot exhaust gases back into the intake manifold, thereby diluting the inducted air/fuel mixture and lowering combustion temperatures to reduce the amount of NOx (oxides of nitrogen) that are created. The amount of exhaust gases re-circulated into the intake manifold, and thus into the cylinders, is proportional to how much the EGR valve is opened and how much intake manifold vacuum is available. Though mostly made up of inert byproducts of the previous combustion event, the exhaust gases partially contain some unburned fuel vapor.
During engine shutdown in an HEV drive cycle, the fuel injectors, VMV, and EGR valves may be flowing at different rates depending on when the shutdown occurs, and thus may contribute fuel vapor and puddle amounts to the intake manifold that vary from one engine shutdown to the next. This, in turn, leads to inconsistent amounts of residual fuel left in the intake manifold from one subsequent engine restart to the next. Because of the many engine shutdowns and starts in an HEV, it is important to minimize the amount of tailpipe emissions during these events.
Nevertheless, with an inconsistent amount of residual fuel vapor and puddles, it becomes difficult to deliver the proper amount of fuel through the injectors from one engine start to the next during the course of a drive cycle. Thus, tailpipe emissions may vary from one engine start to the next during a drive cycle.
A controlled engine shutdown routine can also reduce evaporative emissions following a “key-off” engine (and vehicle) shutdown at the end of a drive cycle. One significant contributor to evaporative emissions in conventional vehicles during a “soak” (i.e., the time between drive cycles where the vehicle is inactive and the engine is off) is residual fuel vapor that migrates to the atmosphere from the intake manifold through the vehicle's air induction system. By reducing the residual fuel from the intake manifold, evaporative emissions can be reduced during the vehicle “key-off” soak periods following a drive cycle.
To accomplish this, a “power sustain” function is needed to continue to provide power to HEV controllers, ignition system, and fuel system (pump and injectors) for a period of time after “key-off.” This allows the generator to continue to spin the engine (after injectors are shut off) while the spark plugs continue to fire until residual fuel (vapor and liquid) is flushed from the intake manifold into the combustion chamber to be combusted (even if partially), and then moved on into the hot catalytic converter to be converted.
Although controlled engine shutdowns are known in the prior art, no such controlled engine shutdown strategy has been developed for an HEV. U.S. Pat. No. 4,653,445 to Book, et al., discloses a control system for engine protection to different threatening conditions. Examples of such conditions include fire, the presence of combustible gas or fuel, rollover or excessive tilt, low oil pressure, low coolant level, engine overheating, or engine overspeed.
Book's engine shutdown system receives warning signals for fault conditions that initiate engine shutdown. Book also includes a method to divide fault signals into either a fast shutdown response or a delayed shutdown response. This method only applies to convention ICE vehicles.
U.S. Pat. No. 4,574,752 to Reichert, Jr., et al., also discloses an engine shutdown device for a conventional ICE and is particularly suited to stationary engine applications. It describes a controlled timed shutdown to reduce engine wear or system damage if problems arise in an external device powered by the engine. When Reichert's method detects a fault in a peripheral device driven or controlled by the engine, it uses a relay, a fuel shutoff control, an engine throttle control, and a timer to shutdown the engine.
Prior art also reveals other developments to reduce fuel waste, emissions and dieseling during controlled engine shutdown for a conventional ICE. U.S. Pat. No. 4,366,790 to DeBoynton, discloses a by-pass system that stops fuel flow to an engine when combustion is not required. When this normally open by-pass valve is closed during events such as deceleration or engine shutdown, only filtered air at a reduced vacuum is allowed into the engine manifold. This prevents fuel waste. See also generally, U.S. Pat. No. 5,357,935 to Oxley, et al. Other systems have developed to maximize the amount of exhaust gas recirculation when an ICE is switched off to reduce emissions and “dieseling.” U.S. Pat. No. 4,367,720 to Miyoshi, et al.
U.S. Pat. No. 4,312,310 to Chivilo, et al., discloses an emissions prevention control system that stops engine fuel intake during idle conditions or deceleration and continues to spin the ICE with an auxiliary power unit such as an electric motor or hydraulic pressure. The motor keeps the engine spinning to allow subsequent fast start-up when normal driving conditions resume.
Although the prior art discloses engine shutdown systems for conventional ICES, they do not meet the engine shutdown needs of an HEV. Thus, a system is needed that controls HEV engine shutdowns to preserve the HEV goal of reduced emissions.